What is the chemical formula for anhydrous salt?

Answer 1

"Anhydrous" means "without water." What they mean by that is, the item in question hasn't absorbed any, usually from the air. So, pick any salt you like - NaCl is pretty common - and as long as it hasn't absorbed water, it's anhydrous.

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An anhydrous salt doesn't contain water of crystallization.

Example: MgSO4.7H2O is a hydrated salt (in this case a heptahydrate) and MgSO4 is the anhydrous form.

A humid salt is not a hydrate and the term anhydrous is not adequate for a dried substance.

Answer 2

The chemical formula for anhydrous salt is the same as the formula for the hydrated version of the salt, but it does not include water molecules. For example, anhydrous copper sulfate is CuSO4, while hydrated copper sulfate is CuSO4·5H2O.

Answer 3

CuSO4·5H2O (pentahydrate) and CuSO4 6H2O (hexahydrate)